Polymeric sheets have a variety of uses, such as in signage, glazings, thermoforming articles, displays and display substrates, for example. For many of these uses, the heat resistance of the sheet is an important factor. Therefore, a higher melting point and glass transition temperature (T.sub.g) are desirable to provide better heat resistance and greater stability. Further, it is desired that sheets have ultraviolet (UV) and scratch resistance, good tensile strength, high optical clarity and a good impact strength, particularly at low temperatures.
Various polymeric compositions have been used in an attempt to meet all of the above criteria. In particular, polyethylene terephthalate (PET) has been used to form low-cost sheets for many years. However, these PET sheets have poor low temperature impact strength, a low glass transition temperature (T.sub.g) and a high rate of crystallization. Thus, PET sheets cannot be used at low temperatures because of the danger of breakage and they cannot be used at high temperatures because the polymer crystallizes, thereby diminishing optical clarity.
Polycarbonate sheets can be used in applications where a low temperature impact strength is needed, or a high service temperature is required. In this regard, polycarbonate sheets have high impact strengths at low temperatures as well as a high T.sub.g which allows them to be used in high temperature applications. However, polycarbonate has poor solvent resistance, thereby limiting its use in certain applications, and is prone to stress induced cracking. Polycarbonate sheets also provide a greater impact strength than is needed for certain applications, making them costly and inefficient for use.
Thus, a need exists for a sheet material that offers (1) high impact strength at low temperature, (2) a higher service temperature (3) good solvent resistance and (4) a low rate of crystallization.
The diol 1,4:3,6-dianhydro-D-sorbitol, referred to hereinafter as isosorbide, the structure of which is illustrated below, is readily made from renewable resources, such as sugars and starches. For example, isosorbide can be made from D-glucose by hydrogenation followed by acid-catalyzed dehydration. ##STR1##
Isosorbide has been incorporated as a monomer into polyesters that also include terephthaloyl moieties. See, for example, R. Storbeck et al, Makromol. Chem., Vol. 194, pp. 53-64 (1993); R. Storbeck et al, Polymer, Vol. 34, p. 5003 (1993). However, it is generally believed that secondary alcohols such as isosorbide have poor reactivity and are sensitive to acid-catalyzed reactions. See, for example, D. Braun et al., J. Prakt. Chem., Vol. 334, pp. 298-310 (1992). As a result of the poor reactivity, polyesters made with an isosorbide monomer and esters of terephthalic acid are expected to have a relatively low molecular weight. Ballauff et al, Polyesters (Derived from Renewable Sources), Polymeric Materials Encyclopedia, Vol. 8, p. 5892 (1996).
Copolymers containing isosorbide moieties, ethylene glycol moieties, and terephthaloyl moieties have been reported only rarely. A copolymer containing these three moieties, in which the mole ratio of ethylene glycol to isosorbide was about 90:10, was reported in published German Patent Application No.1,263,981 (1968). The polymer was used as a minor component (about 10%) of a blend with polypropylene to improve the dyeability of polypropylene fiber. It was made by melt polymerization of dimethyl terephthalate, ethylene glycol, and isosorbide, but the conditions, which were described only in general terms in the publication, would not have given a polymer having a high molecular weight.
Copolymers of these same three monomers were described again recently, where it was observed that the glass transition temperature Tg of the copolymer increases with isosorbide monomer content up to about 200.degree. C. for the isosorbide terephthalate homopolymer. The polymer samples were made by reacting terephthaloyl dichloride in solution with the diol monomers. This method yielded a copolymer with a molecular weight that is apparently higher than was obtained in the German Patent Application described above but still relatively low when compared against other polyester polymers and copolymers. Further, these polymers were made by solution polymerization and were thus free of diethylene glycol moieties as a product of polymerization. See R. Storbeck, Dissertation, Universitat Karlsruhe (1994); R. Storbeck, et al., J. Appl. Polymer Science, vol. 59, pp.1199-1202 (1996).
U.S. Pat. No. 4,418,174 describes a process for the preparation of polyesters useful as raw materials in the production of aqueous stoving lacquers. The polyesters are prepared with an alcohol and an acid. One of the many preferred alcohols is dianhydrosorbitol. However, the average molecular weight of the polyesters is from 1,000 to 10,000, and no polyester actually containing a dianhydrosorbitol moiety was made.
U.S. Pat. No. 5,179,143 describes a process for the preparation of compression molded materials. Also, described therein are hydroxyl containing polyesters. These hydroxyl containing polyesters are listed to include polyhydric alcohols, including 1,4:3,6-dianhydrosorbitol. Again, however, the highest molecular weights reported are relatively low, i.e., 400 to 10,000, and no polyester actually containing the 1,4:3,6-dianhydrosorbitol moiety was made.
Published P.C.T. applications WO 97114739 and WO 96125449 describe cholesteric and nematic liquid crystalline polyesters that include isosorbide moieties as monomer units. Such polyesters have relatively low molecular weights and are not isotropic.